PRINTING PAPER WITHOUT INK

Abstract

A method for printing a wet web material comprising microfibrillated cellulose, wherein said method comprises the steps of providing an aqueous suspension comprising microfibrillated cellulose; applying said aqueous suspension to a substrate, thus forming a wet web having a moisture content in the range of 5 to 70 weight-%; wherein the method further comprises the step of treating said wet web by heating at least one well-defined portion thereof, whereby the web is provided with a print at the at least one heated portion.

Claims

1. A method for printing a wet web material comprising microfibrillated cellulose, wherein said method comprises the steps of: providing an aqueous suspension comprising microfibrillated cellulose; applying said aqueous suspension to a substrate, thus forming a wet web having a moisture content in the range of 5 to 70 weight-%; wherein the method further comprises the step of: treating said wet web by heating at least one well-defined portion thereof, whereby the web is provided with a print at the at least one heated portion.

2. The method as claimed in claim 1, wherein the method further comprises the step of de-watering or drying the web.

3. The method as claimed in claim 2, wherein the dried web has a basis weight of less than 60 gsm.

4. The method as claimed in claim 2, wherein the dried web has a density of 400-1500 kg/m3.

5. The method as claimed in claim 1 wherein the substrate is a porous wire of a papermaking machine.

6. The method as claimed in claim 1, wherein the well defined portions comprises at least one of a figure and a letter.

7. The method as claimed in claim 1, wherein the heating is performed by using laser.

8. The method as claimed in claim 1, wherein the suspension comprises microfibrillated cellulose in an amount of 60-100 wt-% based on total dry solid content.

9. The method as claimed in claim 1, wherein the moisture content in the wet web is in the range of from 10 to 60 weight-%.

10. The method as claimed in claim 1, wherein the step of heating is performed in an in-line process step.

11. The method as claimed in claim 1, wherein the step of heating is performed in an off-line process step.

12. The method as claimed in claim 1, wherein the method further comprises a step of at least one of treating the surface of the web and coating the web.

13. A paper or paper product, comprising a web, obtainable by the method as claimed in claim 1.

14. The method as claimed in claim 1, wherein the moisture content in the wet web is in the range of from 20 to 50 weight-%.

15. The method as claimed in claim 1, wherein the moisture content in the wet web is in the range of from 25 to 45 weight-%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Embodiments of the present solution will now be described, by way of example, with reference to the accompanying schematic drawings.

[0035] FIG. 1 is a photograph of a laser printed images in a dried material.

[0036] FIG. 2 is a photograph showing the increased height in the z-direction of the laser printed areas.

DESCRIPTION OF EMBODIMENTS

[0037] According to the invention a method for printing a wet web material comprising microfibrillated cellulose is provided.

[0038] The method comprises the steps of applying an aqueous suspension comprising microfibrillated cellulose and applying said aqueous suspension to a substrate, thus forming a wet web having a moisture content in the range of 5 to 70 weight-%. The wet web is then treated by heating well defined portions of the web.

[0039] The heating of the web causes the microfibrillated cellulose in the wet web to swell, and by using a heating method that impacts, i.e. heats up, only very well defined portions of the web, a pattern or a print can be achieved. This means that the heated portions may have an increased profile as seen in a side view (z-direction of the web) than the un-heated parts of the wet web, thus also providing the web with a specific touch feeling.

[0040] The inventive method thus allows for a very well-defined pattern to be printed on the wet web, without using any types of ink or other means of coloring the web.

[0041] The print may be done on and/or in the web. The printing may be done locally on the surface of the web or be incorporated into the web, i.e. also be in the web.

[0042] The heating method may be any suitable type of heating for providing a well-defined or local print on the web. According to one embodiment the heat is provided by a hot surface, e.g. a calender. According to an alternative the heating is performed by hot air, for instance blown at the web by a pressurized air nozzle.

[0043] According to another embodiment the heating is performed by using a laser or a laser beam. The laser beam thus impacts the wet web directly and creates the print or pattern on the web. The strength, or level, of the laser beam may be adjusted to give the desired effect in the wet web.

[0044] A film or web formed from microfibrillated cellulose is strong and it is difficult to tear the web, especially if the web is wet. With low laser energy or high moisture content the laser will not cut the fibers and in the printed area there is hardly any change in tearing. With a low moisture content or high laser energy the laser may cut some of the top fibers. It may thus be possible to also cause pinholes or cuts with the laser. This partial cutting of the web may be used to provide a tear line or opening line, for instance in packages, in connection with the printed area, i.e. both a print and a cutting line may be provided in the web.

[0045] The printing means may be digital (variable) or analogue.

[0046] The print or pattern may be any type of shape or form. According to one embodiment the print comprises a letter or a series of letters, or even Braille letters. According to one embodiment the print comprises a figurative pattern, such as for instance a logotype or a graphic design. The printing may thus be a marking or purely decorative.

[0047] The moisture content of the wet web may be varied, depending on the starting materials, and the desired end product. It is essential that the moisture content is sufficient to allow for the heat treatment to cause the microfibrillated cellulose to swell in such a way that the print becomes visible. This means that the moisture content preferably is above 5 weight-%, but it may be in the range of 5 to 80 weight-%. Alternatively the moisture content is in a range from 10 to 60 weight-%, or from 20 to 50 weight-%, or from 25 to 45 weight-%.

[0048] If the invention is done in-line it is possible to incorporate it into a conventional paper or paperboard making process. According to one embodiment the web is produced in a wet laid process.

[0049] According to one embodiment the method may further comprise a step of treating the surface of the web and/or coating the web.

[0050] By treating the surface is meant that the web may be surface sized, or coated etc. By surface sizing is meant contact coating methods used in paper and paperboard industry. Those are e.g. film press, surface sizing (pound or flooded nip size press), gate roll, Gate roll Inverted coater, Twin HSM applicator, Liquid application system, blade/roll metering with the Bill blade, TwoStream, Blade/Blade metering with the mirrorBlade, VACPLY, or application and metering with a nozzle unit onto paper web (Chapt. 14, Coating and surface sizing technologies, Linnonmaa, J., and Trefz, M., in Pigment coating and surface sizing of paper, Papermaking Science and Technology, Book 11, 2.sup.nd Ed., 2009). In addition, reverse gravure or gravure methods, sizing based on indirect metering onto roll using e.g. spray, spinning or foam deposition may also be included in this definition. Other variations and modifications or combinations of the coating methods, obvious for a person skilled in the art, are also included herein.

[0051] According to one embodiment the wet web may be de-watered or dried subsequent to the heat treatment, to provide a dry or substantially dry paper product. According to one embodiment the wet web may also be laminated, to e.g. a fibrous sheet or film, such as a paper or paperboard, or to a thermoplastic polymer sheet or film.

[0052] The web may also be coated with any conventional coatings, such as dispersion coating or other transparent or semitransparent coatings.

[0053] The product formed through the process may be any type of paper or paperboard product. According to one embodiment the product may be a tissue product. According to one embodiment the product is a woven product. According to another embodiment the product is a non-woven product.

[0054] According to one alternative the product may be a never dried wet web, for instance a wet wipe formed from a web comprising mainly microfibrillated cellulose.

[0055] Microfibrillated cellulose (MFC) shall in the context of the patent application mean a nano scale cellulose particle fiber or fibril with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The liberated fibrils have a diameter less than 100 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods. The smallest fibril is called elementary fibril and has a diameter of approximately 2-4 nm (see e.g. Chinga-Carrasco, G., Cellulose fibres, nanofibrils and microfibrils: The morphological sequence of MFC components from a plant physiology and fibre technology point of view, Nanoscale research letters 2011, 6:417), while it is common that the aggregated form of the elementary fibrils, also defined as microfibril (Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vol 53, No. 3.), is the main product that is obtained when making MFC e.g. by using an extended refining process or pressure-drop disintegration process. Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber).

[0056] There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1-5 wt %) when dispersed in water. The cellulose fiber is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 200 m2/g, or more preferably 50-200 m2/g when determined for a freeze-dried material with the BET method.

[0057] Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment step is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp to be supplied may thus be pre-treated enzymatically or chemically, for example to hydrolyse or swell fiber or reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl (CMC), aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxydation, for example TEMPO), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC or nanofibrillar size or NFC.

[0058] The nanofibrillar cellulose may contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines, or nanocrystalline cellulose or e.g. other chemicals present in wood fibers or in papermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated. MFC is produced from wood cellulose fibers, both from hardwood or softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp including pulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper.

[0059] The above described definition of MFC includes, but is not limited to, the new proposed TAPPI standard W13021 on cellulose nanofibril (CNF) defining a cellulose nanofiber material containing multiple elementary fibrils with both crystalline and amorphous regions, having a high aspect ratio with width of 5-30 nm and aspect ratio usually greater than 50.

Example

[0060] A sheet, i.e. a wet wipe, was prepared in accordance with the method disclosed in SE537517 C2. This wet laid sheet comprising microfibrillated cellulose (MFC), was heat treated, i.e. printed, with a laser and the sheet was subsequently dried under tension after the laser printing. In FIG. 1 the sheet is depicted with a figurative print when is drying. The print can be clearly seen where the sheet was subjected to heating by the laser as well defined portions on the sheet.

[0061] In FIG. 2 a sheet has been provided with a print comprising letters. The increased height of the material in the z-direction of the material, i.e. the swollen portions of the material can be clearly seen in FIG. 2.

[0062] In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.